In recent years, an optical recording medium (hereinafter may be referred to as “phase-change optical disk”, “optical disk”, or “phase-changeable optical recording medium” having a recording layer composed of a phase-change material has been increasingly developed.
Generally, in a phase-change optical disk, a specific groove is formed on a transparent plastic substrate, and a thin film is formed on the groove. As a plastic material of a substrate, a polycarbonate resin is mainly used and an injection molding method may be used for forming a groove of the substrate. The thin film formed on the substrate has a multilayered structure in which a first protective layer, a recording layer, a second protective layer, and a reflective layer are basically formed in this order on the substrate.
The first protective layer and the second protective layer are respectively formed with an oxide, a nitride, a sulfide, and the like. Among these, ZnS—SiO2, a mixture of ZnS and SiO2 are preferable.
For the recording layer, a phase-change material containing SbTe as the main component may be used. Specifically, examples of the phase-change material include Ge—Sb—Te, In—Sb—Te, Ag—In—Sb—Te, Ge—In—Sb—Te, Ge—Sn—Sb—Te, Ge—Te, In—Sb, Ga—Sb, and Ge—Sb.
For the reflective layer, metal materials may be used, however, from the viewpoint of optical properties and thermal conductivity, metal materials such as Al, Ag, Au, Cu, and an alloy thereof may be used. Besides, for the purpose of the improvement of various disk properties, a different layer called an insert layer or an interface layer can be formed in between the above-noted respective layers, or each layer itself can be formed with multiple layers. For forming each of these layers, various layer forming methods such as resistance wire heating method, electron beam evaporation method, sputtering method, CVD method can be used. Among these, the sputtering method is particularly preferable from the viewpoint of mass productivity.
After a multi-layered film of these layers is formed, the multi-layered film is coated with a resin layer by spin-coating to protect a thin film.
In a phase-change optical disk produced in this way, a phase-change material used in a recording layer is in an amorphous state, and the phase-change optical disk is generally subjected to a so-called initialization step in which the recording layer is crystallized. In a typical initialization method for initializing a phase-change optical disk, the disk is irradiated with laser beam from a semiconductor laser with a width of a several micrometers, a length of several tens micrometers to several hundreds micrometers while rotating a disk and moving the laser beam in the radius direction. In many cases, an optical disk is more efficiently irradiated with a laser beam by providing with a focusing function. In the initialized phase-change optical disk, arbitrary amorphous marks can be formed by irradiating the disk with a laser according to an arbitrarily predetermined light emission pattern (recording strategy). Besides, a phase-change optical disk can carry out erasing and recording simultaneously called direct overwrite (DOW) recording. Here, the erasing means crystallizing marks in amorphous state, and the recording means forming marks in amorphous state from crystallized marks.
Recording strategy includes ternary control (Pw>Pe>Pb) of recording power (Pw), erasing power (Pe), and bias power (Pb). By combining these and various pulse widths, marks having a specific mark length are recorded. For a modulation method for recording and reproducing data, there are EFM modulation in CDs and EFM+ modulation used in DVDs. Since these modulation methods employ a mark-edge recording mode, controlling a mark length is very important. Jitter property is generally used for evaluation of controlling the mark length.
These phase-change optical disks are used for CD-RW, DVD+RW, DVD-RW, DVD-RAM, and are widely used for audiovisual application and for recording information in computers. Recently, with high-volume of digital capacity, application of these phase-change optical disks to higher volume HD-DVD, Blu-Ray Disc has also been started. With an increase of such recording capacity, further high speed recording is expected as well.
The high speed recording means recording that can be achieved by primarily increasing the number of rotations of a disk and also means recording at a speed as fast as 8×-speed or more of standard linear velocity of DVDs and 28 m/s or more of linear velocity.
Furthermore, in view of practicality, an optical recording medium having compatibility with optical disk drive apparatuses that have been commercially available, so called downward compatibility, is preferable, and not only high-speed recording but also low speed recording are required.
As described above, GaSb based phase-change materials are known. Patent Literature 1 and 2 respectively disclose high linear velocity recording. However, since there is no specific description with regard to recordable linear velocity range in the Patent Literature mentioned above, these related arts can not achieve the purpose of the present invention that enables recording in a wide linear velocity range.
Under these circumstances, the present inventors experienced the phenomenon that the number of reproducing errors in a phase-change material suitable for high-speed recording was increased in a certain range of recording linear velocities. For example, a report similar to the phenomenon is described in Non Patent Literature 1.
Hereinafter, the description about the phenomenon found by the inventors of the present invention will be described below.
FIG. 1 shows the relation between recording linear velocities of a disk developed for recording DVD+RW 8×-speed and jitter property DOW 10 times recording and PI errors (Parity Inner Error:intra-coding parity error) property. In FIG. 1, horizontal scale represents recording linear velocity, left-side vertical scale represents PI errors, and right-side vertical scale represents DOW 10 Jitter. PI errors means the above-noted reproducing errors. Besides, for recording conditions, conditions under which the jitter property is optimum are used.
As can be seen from FIG. 1, the jitter property of the disk at a speed of 3× to 8× shows excellent results i.e. about 9% or less, whereas the number of PI errors is drastically increased in the range of 4×-speed to 7×-speed which are intermediate linear velocities. It is considered that when the number of PI errors is 280 or more, particularly 350 or more, it causes problems in practical use.
In FIG. 1 the number of PI errors far more than the above noted values are shown, and it is apparent that jitter property greatly inversely relates to error property. In FIG. 1, similar phenomena can be verified without depending on the number of DOW times, although the results vary to some extent.
In order to examine the detail of this phenomenon, for a disk used in FIG. 1, a single pattern that 3T mark and 3T space are aligned alternately is recorded using the recording strategy shown in FIG. 2. A pattern diagram of the recording mark shapes is given in FIG. 3A. Horizontal scale of FIG. 2 represents time, and vertical scale represents signal intensity. The shape of A mark shape is the observation result using a transmission electron microscope.
The mark A and the mark C in FIG. 3A are normal recording marks whereas the mark B is an abnormal mark in which crystal occurs within the mark.
FIG. 3B shows a reproducing signal of a recording mark. A dashed line is the case when a recording mark is normal, whereas when a recording mark has crystal like the mark B, a reproducing signal may be distorted like shown as a solid line. As a result, binarized signal is like the one in FIG. 3C, abnormal mark B with crystal is only reproduced as shorter than normal 3T mark. Here, only data of recording 3T single pattern is shown, and it is verified that this problem also occurs in other single patterns.
The results of such signal measured by TIA (Time Interval Analyzer) are given in FIG. 4 as a pattern diagram. FIG. 4 shows the distribution of the abnormal mark and the normal mark, horizontal scale represents a mark length and vertical scale (logarithmic axis) represents the number of marks.
As shown in FIG. 4, it can be divided into components showing a normal distribution centered on 3T and components which are distributed in regions shorter than 3T. The components distributed in the regions smaller than 3T correspond to the number of abnormal marks existing in recording marks, which will cause PI errors.
As examples that crystal affects amorphous marks, the following (1) to (3) are known.
(1) Partial recrystallization of marks by residual heat, also called cross erase is reported (see Patent Literature 3).
(2) It is reported that unerased marks are generated due to insufficient crystallization at high-speed recording (see Patent Literature 4).
(3) It is reported that crystals are deposited around amorphous marks in DOW recording multiple times (see Patent Literature 5 to Patent Literature 7).
It is found that the phenomenon is different from conventionally known phenomenon from the viewpoint that the phenomenon is does not depend on the number of DOW times, crystal is not generated in all the amorphous marks, the number of reproducing errors is significantly increased regardless of favorable jitter property, and crystal exists within marks, not around the marks.
Furthermore, as recording density increases to the extent of DVDs, it is thought that the existence of crystal as described above in recording mark leads to increases in reproducing errors, and it is expected that this may cause a significant problem in a phase-change optical disk using a blue laser which enables higher density recording.
The inventors further examined the phenomenon in which errors are increased in the intermediate linear velocity and found out that the phenomenon depends heavily on a crystallization rate of a phase-change material.
FIG. 5 shows the relation between crystallization rates of various phase-change materials and the number of abnormal marks of 3T mark. Horizontal scale in FIG. 5 represents a crystallization rate and vertical scale represents the number of abnormal marks.
The number of abnormal marks is the total obtained in the evaluation of TIA (Time Interval Analyzer) and is the standardized one that the number of abnormal marks existing in shorter areas than those of 3T. As can be seen from FIG. 5, the number of abnormal marks is increased in high-crystallization rate area bordered by a specific crystallization rate. For this reason, it is necessary to limit the crystallization rate to a value lower than a specific value in order to control abnormal marks.
Although Patent Literature 8 to 10 proposes the invention using InSbx as materials for a recording layer, there is neither disclosure nor indication about using any one of zinc oxide, indium oxide, tin oxide, and a mixture thereof as a protective layer material.
In Patent Literature 9, although there is a description with regard to standard clock frequency, recording conditions, the relation between recording linear velocity and recording density (the shortest mark length) is not clear.
Patent Literature 10 discloses that recording principle is changes induced between crystallized marks, however, there is neither disclosure nor indication about change induced between crystallized marks and amorphous marks.
Although Patent Literature 11 to 16 disclose that the main component of the second protective layer is at least one selected from zinc oxides, indium oxides, and tin oxides, there are the following problems.
In Patent Literature 11, there is no description about the composition with regard to the combination with InSbx so that the purpose of the present invention can not be accomplished.
In Patent Literature 12, there is only an indication about the composition of combining 2 kinds of protective layer materials so that the purpose of the present invention also can not be accomplished.
The invention described in Patent Literature 13 uses different recording layer materials and is a totally different technology from the composition of the present invention.
In Patent Literature 14 to 15, there is no description of composition with regard to the combination with InSbx so that the purpose of the present invention can not be accomplished.
In Patent Literature 16, there is no description of composition with regard to the combination with InSbx and the position of protective layer materials is different so that the purpose of the present invention can not be accomplished.
In order not only to resolve the above-noted issues but also to attain high-speed recording, it is necessary to make crystallization rate faster. The reason is generally thought as that if the crystallization rate is slower than the recording linear velocity, crystallization at the overwriting can not be fulfilled so that sufficient erasing can not be done.
For DVD+RW optical disks for 8×-speed recording, although low speed recording to 3.3×-speed is possible by optimizing a recording method and further employment of additional materials and layer structure, there is a problem that it is difficult to achieve wider recording linear velocities in consideration of further high speed recording and downward compatibility.    [Patent Literature 1] Japanese Patent Application Laid-Open (JP-A) No. 2005-145061    [Patent Literature 2] Japanese Patent Application Laid-Open (JP-A) No. 2004-203011    [Patent Literature 3] Japanese Patent Application Laid-Open (JP-A) No. 2004-164850    [Patent Literature 4] Japanese Patent Application Laid-Open (JP-A) No. 2004-164849    [Patent Literature 5] Japanese Patent Application Laid-Open (JP-A) No. 4-286683    [Patent Literature 6] Japanese Patent Application Laid-Open (JP-A) No. 6-103609    [Patent Literature 7] Japanese Patent (JP-B) No. 3474714    [Patent Literature 8] Japanese Patent Application Laid-Open (JP-A) No. 2005-193663    [Patent Literature 9] Japanese Patent Application Laid-Open (JP-A) No. 2002-347341    [Patent Literature 10] Japanese Patent Application Publication (JP-B) No. 3-52651    [Patent Literature 11] Japanese Patent Application Laid-Open (JP-A) No. 2005-190642    [Patent Literature 12] Japanese Patent Application Laid-Open (JP-A) No. 5-101442    [Patent Literature 13] Japanese Patent (JP-B) No. 2559803    [Patent Literature 14] Japanese Patent Application Laid-Open (JP-A) No. 5-159362    [Patent Literature 15] Japanese Patent Application Laid-Open (JP-A) No. 11-185294    [Patent Literature 16] Japanese Patent Application Laid-Open (JP-A) No. 5-208559    [Non Patent Literature 1] H. Spruit et al.: High Speed DVD+RW Recording, ISOM/ODS'05 Tech. Dig. (2005) TuC1